Seldom has an image so clearly marked the turning of the world. One of man's mightiest structures collapses into an immense white blossom of churning, roiling dust, metamorphosing in 14 seconds from hundred-story giant of the earth into towering white plume reaching to heaven. The demise of the World Trade Center gave us an image as newborn to the world of sight as the mushroom cloud must have appeared to those who first cast eyes on it. I recall vividly the seconds flowing by as I sat gaping at the screen, uncomprehending and unbelieving, while Peter Jennings's urbane, perfectly modulated voice murmured calmly on about flights being grounded, leaving unacknowledged and unexplained — unconfirmed — the incomprehensible scene unfolding in real time before our eyes. "˜"˜Hang on there a second,'' the famously unflappable Jennings finally stammered — the South Tower had by now vanished into a boiling caldron of white smoke — "˜"˜I just want to check one thing . . . because . . . we now have. . . . What do we have? We don't . . . ?'' Marveling later that "˜"˜the most powerful image was the one I actually didn't notice while it was occurring,'' Jennings would say simply that "˜"˜it was beyond our imagination.''

Looking back from this moment, precisely four years later, it still seems almost inconceivable that 10 men could have done that — could have brought those towers down. Could have imagined doing what was "˜"˜beyond our imagination.'' When a few days later, the German composer Karlheinz Stockhausen remarked that this was "˜"˜the greatest work of art in the history of the cosmos,'' I shared the anger his words called forth but couldn't help sensing their bit of truth: "˜"˜What happened there — spiritually — this jump out of security, out of the everyday, out of life, that happens sometimes poco a poco in art.'' No "˜"˜little by little'' here: however profoundly evil the art, the sheer immensity and inconceivability of the attack had forced Americans instantaneously to "˜"˜jump out of security, out of the everyday, out of life'' and had thrust them through a portal into a strange and terrifying new world, where the inconceivable, the unimaginable, had become brutally possible.

In the face of the unimaginable, small wonder that leaders would revert to the language of apocalypse, of crusade, of "˜"˜moral clarity.'' Speaking at the National Cathedral just three days after the attacks, President Bush declared that while "˜"˜Americans do not yet have the distance of history . . . our responsibility to history is already clear: to answer these attacks and rid the world of evil.'' Astonishing words — imaginable, perhaps, only from an American president, leading a people given naturally in times of crisis to enlisting national power in the cause of universal redemption. "˜"˜The enemy is not a single political regime or person or religion or ideology,'' declared the National Security Strategy of the United States of America for 2002. "˜"˜The enemy is terrorism — premeditated, politically motivated violence perpetrated against innocents.'' Not Islamic terrorism or Middle Eastern terrorism or even terrorism directed against the United States: terrorism itself. "˜"˜Declaring war on "˜terror,''' as one military strategist later remarked to me, "˜"˜is like declaring war on air power.'' It didn't matter; apocalypse, retribution, redemption were in the air, and the grandeur of the goal must be commensurate with the enormity of the crime. Within days of the attacks, President Bush had launched a "˜"˜global war on terror.''

Today marks four years of war. Four years after the attack on Pearl Harbor, U.S. troops ruled unchallenged in Japan and Germany. During those 48 months, Americans created an unmatched machine of war and decisively defeated two great enemies.

How are we to judge the global war on terror four years on? In this war, the president had warned, "˜"˜Americans should not expect one battle but a lengthy campaign.'' We could expect no "˜"˜surrender ceremony on a deck of a battleship,'' and indeed, apart from the president's abortive attempt on the U.S.S. Lincoln to declare victory in Iraq, there has been none. Failing such rituals of capitulation, by what "˜"˜metric'' — as the generals say — can we measure the progress of the global war on terror?

Four years after the collapse of the towers, evil is still with us and so is terrorism. Terrorists have staged spectacular attacks, killing thousands, in Tunisia, Bali, Mombasa, Riyadh, Istanbul, Casablanca, Jakarta, Madrid, Sharm el Sheik and London, to name only the best known. Last year, they mounted 651 "˜"˜significant terrorist attacks,'' triple the year before and the highest since the State Department started gathering figures two decades ago. One hundred ninety-eight of these came in Iraq, Bush's "˜"˜central front of the war on terror'' — nine times the year before. And this does not include the hundreds of attacks on U.S. troops. It is in Iraq, which was to serve as the first step in the "˜"˜democratization of the Middle East,'' that insurgents have taken terrorism to a new level, killing well over 4,000 people since April in Baghdad alone; in May, Iraq suffered 90 suicide-bombings. Perhaps the "˜"˜shining example of democracy'' that the administration promised will someday come, but for now Iraq has become a grotesque advertisement for the power and efficacy of terror.

As for the "˜"˜terrorist groups of global reach,'' Al Qaeda, according to the president, has been severely wounded. "˜"˜We've captured or killed two-thirds of their known leaders,'' he said last year. And yet however degraded Al Qaeda's operational capacity, nearly every other month, it seems, Osama bin Laden or one of his henchmen appears on the world's television screens to expatiate on the ideology and strategy of global jihad and to urge followers on to more audacious and more lethal efforts. This, and the sheer number and breadth of terrorist attacks, suggest strongly that Al Qaeda has now become Al Qaedaism — that under the American and allied assault, what had been a relatively small, conspiratorial organization has mutated into a worldwide political movement, with thousands of followers eager to adopt its methods and advance its aims. Call it viral Al Qaeda, carried by strongly motivated next-generation followers who download from the Internet's virtual training camp a perfectly adequate trade-craft in terror. Nearly two years ago, Secretary of Defense Donald H. Rumsfeld, in a confidential memorandum, posed the central question about the war on terror: "˜"˜Are we capturing, killing or deterring and dissuading more terrorists every day than the madrassas and the radical clerics are recruiting, training and deploying against us?'' The answer is clearly no. "˜"˜We have taken a ball of quicksilver,'' says the counterinsurgency specialist John Arquilla, "˜"˜and hit it with a hammer.''

What has helped those little bits of quicksilver grow and flourish is, above all, the decision to invade and occupy Iraq, which has left the United States bogged down in a brutal, highly visible counterinsurgency war in the heart of the Arab world. Iraq has become a training ground that will temper and prepare the next generation of jihadist terrorists and a televised stage from which the struggle of radical Islam against the "˜"˜crusader forces'' can be broadcast throughout the Islamic world. "˜"˜Islamic extremists are exploiting the Iraqi conflict to recruit new anti-U.S. jihadists,'' Porter J. Goss, director of the C.I.A., told the Senate in February. "˜"˜These jihadists who survive will leave Iraq experienced in, and focused on, acts of urban terrorism. They represent a potential pool of contacts to build transnational terrorist cells, groups and networks in Saudi Arabia, Jordan and other countries.''

As the Iraq war grows increasingly unpopular in the United States — scarcely a third of Americans now approve of the president's handling of the war, and 4 in 10 think it was worth fighting — and as more and more American leaders demand that the administration "˜"˜start figuring out how we get out of there'' (in the words of Senator Chuck Hagel, a Republican), Americans confront a stark choice: whether to go on indefinitely fighting a politically self-destructive counterinsurgency war that keeps the jihadists increasingly well supplied with volunteers or to withdraw from a post-Saddam Hussein Iraq that remains chaotic and unstable and beset with civil strife and thereby hand Al Qaeda and its allies a major victory in the war on terror's "˜"˜central front.''

Four years after we watched the towers fall, Americans have not succeeded in "˜"˜ridding the world of evil.'' We have managed to show ourselves, our friends and most of all our enemies the limits of American power. Instead of fighting the real war that was thrust upon us on that incomprehensible morning four years ago, we stubbornly insisted on fighting a war of the imagination, an ideological struggle that we defined not by frankly appraising the real enemy before us but by focusing on the mirror of our own obsessions. And we have finished — as the escalating numbers of terrorist attacks, the grinding Iraq insurgency, the overstretched American military and the increasing political dissatisfaction at home show — by fighting precisely the kind of war they wanted us to fight.

II

Facing what is beyond imagination, you find sense in the familiar. Standing before Congress on Sept. 20, 2001, George W. Bush told Americans why they had been attacked. "˜"˜They hate our freedoms,'' the president declared. "˜"˜Our freedom of speech, our freedom to vote and assemble and disagree with each other.'' As for Al Qaeda's fundamentalist religious mission: "˜"˜We are not deceived by their pretenses to piety. We have seen their kind before. They are the heirs of all the murderous ideologies of the 20th century. By sacrificing human life to serve their radical visions — by abandoning every value except the will to power — they follow in the path of fascism, and Nazism, and totalitarianism. And they will follow that path all the way, to where it ends: in history's unmarked grave of discarded lies.''

Stirring words, and effective, for they domesticated the unthinkable in the categories of the accustomed. The terrorists are only the latest in a long line of "˜"˜evildoers.'' Like the Nazis and the Communists before them, they are Americans' evil twins: tyrants to our free men, totalitarians to our democrats. The world, after a confusing decade, had once again split in two. However disorienting the horror of the attacks, the "˜"˜war on terror'' was simply a reprise of the cold war. As Harry S. Truman christened the cold war by explaining to Americans how, "˜"˜at the present moment in world history, nearly every nation must choose between alternative ways of life,'' George W. Bush declared his global war on terror by insisting that "˜"˜every nation, in every region, now has a decision to make. Either you are with us, or you are with the terrorists.'' The echo, as much administration rhetoric since has shown, was not coincidental. Terrorists, like Communists, despised America not because of what our country did but because of who we are. Hating "˜"˜our values'' and "˜"˜our freedoms,'' the evildoers were depicted as deeply irrational and committed to a nihilistic philosophy of obliteration, reawakening for Americans the sleeping image of the mushroom cloud. "˜"˜This is not aimed at our policies,'' Henry Kissinger intoned. "˜"˜This is aimed at our existence.''

Such rhetoric not only fell easily on American ears. It provided a familiar context for a disoriented national-security bureaucracy that had been created to fight the cold war and was left, at its ending, without clear purpose. "˜"˜Washington policy and defense cultures still seek out cold-war models,'' as members of the Defense Science Board, a Defense Department task force commissioned to examine the war on terror, observed in a report last year. "˜"˜With the surprise announcement of a new struggle, the U.S. government reflexively inclined toward cold-war-style responses to the new threat, without a thought or a care as to whether these were the best responses to a very different strategic situation.''

Al Qaeda was not the Nazis or the Soviet Communists. Al Qaeda controlled no state, fielded no regular army. It was a small, conspiratorial organization, dedicated to achieving its aims through guerrilla tactics, notably a kind of spectacular terrorism carried to a level of apocalyptic brutality the world had not before seen. Mass killing was the necessary but not the primary aim, for the point of such terror was to mobilize recruits for a political cause — to move sympathizers to act — and to tempt the enemy into reacting in such a way as to make that mobilization easier. And however extreme and repugnant Al Qaeda's methods, its revolutionary goals were by no means unusual within Islamist opposition groups throughout the Muslim world. "˜"˜If there is one overarching goal they share,'' wrote the authors of the Defense Science Board report, "˜"˜it is the overthrow of what Islamists call the "˜apostate' regimes: the tyrannies of Egypt, Saudi Arabia, Pakistan, Jordan and the gulf states. . . . The United States finds itself in the strategically awkward — and potentially dangerous — situation of being the longstanding prop and alliance partner of these authoritarian regimes. Without the U.S., these regimes could not survive. Thus the U.S. has strongly taken sides in a desperate struggle that is both broadly cast for all Muslims and country-specific.''

The broad aim of the many-stranded Salafi movement, which includes the Muslim Brotherhood of Egypt and the Wahhabis of Saudi Arabia and of which Al Qaeda is one extreme version, is to return Muslims to the ancient ways of pure Islam — of Islam as it was practiced by the Prophet Muhammad and his early followers in the seventh century. Standing between the more radical Salafi groups and their goal of a conservative Islamic revolution are the "˜"˜apostate regimes,'' the "˜"˜idolators'' now ruling in Riyadh, Cairo, Amman, Islamabad and other Muslim capitals. All these authoritarian regimes oppress their people: on this point Al Qaeda and those in the Bush administration who promote "˜"˜democratization in the Arab world'' agree. Many of the Salafists, however, see behind the "˜"˜near enemies'' ruling over them a "˜"˜far enemy'' in Washington, a superpower without whose financial and military support the Mubarak regime, the Saudi royal family and the other conservative autocracies of the Arab world would fall before their attacks. When the United States sent hundreds of thousands of American troops to Saudi Arabia after Saddam Hussein invaded Kuwait, Al Qaeda seized on the perfect issue: the "˜"˜far enemy'' had actually come and occupied the Land of the Two Holy Places and done so at the shameful invitation of the "˜"˜near enemy'' — the corrupt Saudi dynasty. As bin Laden observed of the Saudis in his 1996 "˜"˜Declaration of Jihad'': "˜"˜This situation is a curse put on them by Allah for not objecting to the oppressive and illegitimate behavior and measures of the ruling regime: ignoring the divine Shariah law; depriving people of their legitimate rights; allowing the Americans to occupy the Land of the Two Holy Places.''

But how to "˜"˜re-establish the greatness of this Ummah'' — the Muslim people — "˜"˜and to liberate its occupied sanctities''? On this bin Laden is practical and frank: because of "˜"˜the imbalance of power between our armed forces and the enemy forces, a suitable means of fighting must be adopted, i.e., using fast-moving light forces that work under complete secrecy. In other words, to initiate a guerrilla warfare.'' Such warfare, depending on increasingly spectacular acts of terrorism, would be used to "˜"˜prepare and instigate the Ummah . . . against the enemy.'' The notion of "˜"˜instigation,'' indeed, is critical, for the purpose of terror is not to destroy your enemy directly but rather to spur on your sleeping allies to enlightenment, to courage and to action. It is a kind of horrible advertisement, meant to show those millions of Muslims who sympathize with Al Qaeda's view of American policy that something can be done to change it.

III

Fundamentalist Islamic thought took aim at America's policies, not at its existence. Americans tend to be little interested in these policies or their history and thus see the various Middle East cataclysms of the last decades as sudden, unrelated explosions lighting up a murky and threatening landscape, reinforcing the sense that the 9/11 attacks were not only deadly and appalling but also irrational, incomprehensible: that they embodied pure evil. The central strand of American policy — unflinching support for the conservative Sunni regimes of the Persian Gulf — extends back 60 years, to a legendary meeting between Franklin D. Roosevelt and King Saud aboard an American cruiser in the Great Bitter Lake in Egypt. The American president and the Saudi king agreed there on a simple bond of interest: the Saudis, rulers over a sparsely populated but incalculably wealthy land, would see their power guaranteed against all threats, internal and external. In return, the United States could count on a stable supply of oil, developed and pumped by American companies. This policy stood virtually unthreatened for more than three decades.

The eruption of Iran's Islamic revolution in 1978 dealt a blow to this compact of interests and cast in relief its central contradictions. The shah, who owed his throne to a covert C.I.A. intervention that returned him to power in 1953, had been a key American ally in the gulf, and the Islamic revolution that swept him from power showed at work what was to become a familiar dynamic: "˜"˜friendly'' autocrats ruling over increasingly impatient and angry peoples who evidence resentment if not outright hostility toward the superpower ally, in whom they see the ultimate source of their own repression.

Iran's Islamic revolution delivered a body blow to the Middle East status quo not unlike that landed by the French Revolution on the European autocratic order two centuries before; it was ideologically aggressive, inherently expansionist and deeply threatening to its neighbors — in this case, to the United States' Sunni allies, many of whom had substantial Shia minorities, and to Iraq, which, though long ruled by Sunnis, had a substantial Shia majority. Ayatollah Khomeini's virulent and persistent calls for Saddam Hussein's overthrow, and the turmoil that had apparently weakened the Iranian armed forces, tempted Saddam Hussein to send his army to attack Iran in 1980. American policy makers looked on this with favor, seeing in the bloody Iran-Iraq war the force that would blunt the revolutionary threat to America's allies. Thus President Reagan sent his special envoy Donald Rumsfeld to Baghdad in 1983 to parlay with Hussein, and thus the administration supported the dictator with billions of dollars of agricultural credits, supplied the Iraqis with hundreds of millions of dollars in advanced weaponry through Egypt and Saudi Arabia and provided Hussein's army with satellite intelligence that may have been used to direct chemical weapons against the massed infantry charges of Iranian suicide brigades.

The Iraqis fought the Iranians to a standstill but not before ripples from Iran's revolution threatened to overwhelm American allies, notably the Saudi dynasty, whose rule was challenged by radicals seizing control of the Grand Mosque in Mecca in November 1979, and the Egyptian autocracy, whose ruler, Anwar el-Sadat, was assassinated by Islamists as he presided over a military parade in October 1981. The Saudis managed to put down the revolt, killing hundreds. The Egyptians, under Hosni Mubarak, moved with ruthless efficiency to suppress the Islamists, jailing and torturing thousands, among them Osama bin Laden's current deputy, Ayman al-Zawahiri. Merciless repression by both autocracies' effective security services led thousands to flee abroad.

Many went to Afghanistan, which the Soviet Red Army occupied in 1979 to prop up its own tottering client, then under threat from Islamic insurgents — mujahedeen, or "˜"˜holy warriors,'' who were being armed by the United States. "˜"˜It was July 3, 1979, that President Carter signed the first directive for secret aid to the opponents of the pro-Soviet regime in Kabul,'' Zbigniew Brzezinski, Carter's national security adviser, recalled in 1998. "˜"˜And that very day, I wrote a note to the president in which I explained to him that in my opinion this aid was going to induce a Soviet military intervention.'' It was a strategy of provocation, for the gambit had the effect of "˜"˜drawing the Russians into the Afghan trap. . . . The day that the Soviets officially crossed the border, I wrote to President Carter: We now have the opportunity of giving to the U.S.S.R. its Vietnam War.''

If, to the Americans, supporting the Afghan mujahedeen seemed an excellent way to bleed the Soviet Union, to the Saudis and other Muslim regimes, supporting a "˜"˜defensive jihad'' to free occupied Muslim lands was a means to burnish their tarnished Islamic credentials while exporting a growing and dangerous resource (frustrated, radical young men) so they would indulge their taste for pious revolution far from home. Among the thousands of holy warriors making this journey was the wealthy young Saudi Osama bin Laden, who would set up the Afghan Services Bureau, a "˜"˜helping organization'' for Arab fighters that gathered names and contact information in a large database — or "˜"˜qaeda'' — which would eventually lend its name to an entirely new organization. Though the Afghan operation was wildly successful, as judged by its American creators — "˜"˜What is most important to the history of the world?'' Brzezinski said in 1998, "˜"˜some stirred-up Muslims or the liberation of Central Europe and the end of the cold war?'' — it had at least one unexpected result: it created a global jihad movement, led by veteran fighters who were convinced that they had defeated one superpower and could defeat another.

The present jihad took shape in the backwash of forgotten wars. After the Soviet Army withdrew in defeat, the United States lost interest in Afghanistan, leaving the mujahedeen forces to battle for the ruined country in an eight-year blood bath from which the Taliban finally emerged victorious. In the gulf, after eight years of fantastically bloody combat, Saddam Hussein forced the Iranians to sign a cease-fire, a "˜"˜victory'' that left his regime heavily armed, bloodied and bankrupt. To pay for his war, Hussein had borrowed tens of billions of dollars from the Saudis, Kuwaitis and other neighbors, and he now demanded that these debts be forgiven — he had incurred them, as he saw it, defending the lenders from Khomeini — and that oil prices be raised. The Kuwaitis' particularly aggressive refusal to do either led Hussein, apparently believing that the Americans would accept a fait accompli, to invade and annex the country.

The Iraqi Army flooding into Kuwait represented, to bin Laden, the classic opportunity. He rushed to see the Saudi leaders, proposing that he defend the kingdom with his battle-tested corps of veteran holy warriors. The Saudis listened patiently to the pious young man — his father, after all, had been one of the kingdom's richest men — but did not take him seriously. Within a week, King Fahd had agreed to the American proposal, carried by Richard Cheney, then the secretary of defense, to station American soldiers — "˜"˜infidel armies'' — in the Land of the Two Holy Places. This momentous decision led to bin Laden's final break with the Saudi dynasty.

The American presence, and the fatal decision to leave American forces stationed in Saudi Arabia as a trip wire or deterrent even after Hussein had been defeated, provided bin Laden with a critical propaganda point, for it gave to his worldview, of a Muslim world under relentless attack, and its central argument, that the "˜"˜unjust and renegade ruling regimes'' of the Islamic world were in fact "˜"˜enslaved by the United States,'' a concrete and vivid reality. The "˜"˜near enemies'' and their ruthless security services had proved resistant to direct assault, and the time had come to confront directly the one antagonist able to bring together all the jihadists in a single great battle: the "˜"˜far enemy'' across the sea.

IV

The deaths of nearly 3,000 people, the thousands left behind to mourn them, the great plume hanging over Lower Manhattan carrying the stench of the vaporized buildings and their buried dead: mass murder of the most abominable, cowardly kind appears to be so at the heart of what happened on this day four years ago that it seems beyond grotesque to remind ourselves that for the attackers those thousands of dead were only a means to an end. Not the least disgusting thing about terrorism is that it makes objects of human beings, makes use of them, exploits their deaths as a means to accomplish something else: to send a message, to force a concession, to advertise a cause. Though such cold instrumentality is not unknown in war — large-scale bombing of civilians, "˜"˜terror bombing,'' as it used to be known, does much the same thing — terrorism's ruthless and intimate randomness seems especially appalling.

Terror is a way of talking. Those who employed it so unprecedentedly on 9/11 were seeking not just the large-scale killing of Americans but to achieve something by means of the large-scale killing of Americans. Not just large-scale, it should be added: spectacular.

The asymmetric weapons that the 19 terrorists used on 9/11 were not only the knives and box cutters they brandished or the fuel-laden airliners they managed to commandeer but, above all, that most American of technological creations: the television set. On 9/11, the jihadists used this weapon with great determination and ruthlessness to attack the most powerful nation in the history of the world at its point of greatest vulnerability: at the level of spectacle. They did it by creating an image, to repeat Peter Jennings's words, "˜"˜beyond our imagination.''

The goal, first and foremost, was to diminish American prestige — showing that the superpower could be bloodied, that for all its power, its defeat was indeed conceivable. All the major attacks preceding 9/11 attributed at least in part to Al Qaeda — the shooting down of U.S. Army helicopters in Mogadishu in 1993, the truck-bombing of American military housing at Khobar in 1996, the car-bombing of the American embassies in Nairobi and Dar es Salaam in 1998, the suicide-bombing of the U.S.S. Cole in Aden in 2000 — were aimed at the same goal: to destroy the aura of American power. Power, particularly imperial power, rests not on its use but on its credibility; U.S. power in the Middle East depends not on ships and missiles but on the certainty that the United States is invincible and stands behind its friends. The jihadis used terrorism to create a spectacle that would remove this certainty. They were by no means the first guerrilla group to adopt such a strategy. "˜"˜History and our observation persuaded us,'' recalled Menachem Begin, the future Israeli prime minister who used terror with great success to drive the British out of Palestine during the mid-1940's, "˜"˜that if we could succeed in destroying the government's prestige in Eretz Israel, the removal of its rule would follow automatically. Thenceforward, we gave no peace to this weak spot. Throughout all the years of our uprising, we hit at the British government's prestige, deliberately, tirelessly, unceasingly.'' In its most spectacular act, in July 1946, the Irgun guerrilla forces led by Begin bombed the King David Hotel, killing 91 people, most of them civilians.

The 9/11 attacks were a call to persuade Muslims who might share bin Laden's broad view of American power to sympathize with, support or even join the jihad he had declared against the "˜"˜far enemy.'' "˜"˜Those young men,'' bin Laden said of the terrorists two months after the attacks, "˜"˜said in deeds, in New York and Washington, speeches that overshadowed all other speeches made everywhere else in the world. The speeches are understood by both Arabs and non-Arabs — even by Chinese. . . . [I]n Holland, at one of the centers, the number of people who accepted Islam during the days that followed the operations were more than the people who accepted Islam in the last 11 years.'' To this, a sheik in a wheelchair shown in the videotape replies: "˜"˜Hundreds of people used to doubt you, and few only would follow you until this huge event happened. Now hundreds of people are coming out to join you.'' Grotesque as it is to say, the spectacle of 9/11 was meant to serve, among other things, as an enormous recruiting poster.

But recruitment to what? We should return here to the lessons of Afghanistan, not only the obvious one of the defeat of a powerful Soviet Army by guerrilla forces but the more subtle one taught by the Americans, who by clever use of covert aid to the Afghan resistance tempted the Soviets to invade the country and thereby drew "˜"˜the Russians into an Afghan trap.'' Bin Laden seems to have hoped to set in motion a similar strategy. According to a text attributed to Saif al-Adel, a former Egyptian Army colonel now generally identified as bin Laden's military chief, "˜"˜the ultimate objective was to prompt'' the United States "˜"˜to come out of its hole'' and take direct military action in an Islamic country. "˜"˜What we had wished for actually happened. It was crowned by the announcement of Bush Jr. of his crusade against Islam and Muslims everywhere.'' ("˜"˜This is a new kind of evil,'' the president said five days after the attacks, "˜"˜and we understand . . . this crusade, this war on terrorism, is going to take a while.'')

The 9/11 attacks seem to have been intended at least in part to provoke an overwhelming American response: most likely an invasion of Afghanistan, which would lead the United States, like the Soviet Union before it, into an endless, costly and politically fatal quagmire. Thus, two days before the attacks, Qaeda agents posing as television journalists taping an interview murdered Ahmed Shah Massoud, the charismatic leader of the Northern Alliance, with a bomb concealed in a video camera — apparently a pre-emptive strike intended to throw into confusion the United States' obvious ally in the coming invasion of Afghanistan.

For the jihadists, luring the Americans into Afghanistan would accomplish at least two things: by drawing the United States into a protracted guerrilla war in which the superpower would occupy a Muslim country and kill Muslim civilians — with the world media, including independent Arab networks like Al Jazeera, broadcasting the carnage — it would leave increasingly isolated those autocratic Muslim regimes that depended for their survival on American support. And by forcing the United States to prosecute a long, costly and inconclusive guerrilla war, it would severely test, and ultimately break, American will, leading to a collapse of American prestige and an eventual withdrawal — first, physically, from Afghanistan and then, politically, from the "˜"˜apostate regimes'' in Riyadh, Cairo and elsewhere in the Islamic world.

In his "˜"˜Declaration of Jihad'' in 1996, bin Laden focused on American political will as the United States' prime vulnerability, the enemy's "˜"˜center of gravity'' that his guerrilla war must target and destroy. "˜"˜The defense secretary of the crusading Americans had said that "˜the explosions at Riyadh and Al-Khobar had taught him one lesson: that is, not to withdraw when attacked by cowardly terrorists.' We say to the defense secretary, Where was this false courage of yours when the explosion in Beirut took place in 1983?

"˜"˜But your most disgraceful case was in Somalia. . . . When tens of your soldiers were killed in minor battles and one American pilot was dragged in the streets of Mogadishu, you left the area carrying disappointment, humiliation, defeat and your dead with you. . . . The extent of your impotence and weaknesses became very clear.''

In Afghanistan, bin Laden would be disappointed. The U.S. military initially sent in no heavy armor but instead restricted the American effort to aerial bombardment in support of several hundred Special Operations soldiers on the ground who helped lead the Northern Alliance forces in a rapid advance. Kabul and other cities quickly fell. America was caught in no Afghan quagmire, or at least not in the sort of protracted, highly televisual bloody mess bin Laden had envisioned. But bin Laden and his senior leadership, holed up in the mountain complex of Tora Bora, managed to survive the bombing and elude the Afghan forces that the Americans commissioned to capture them. During the next months and years, as the United States and its allies did great damage to Al Qaeda's operational cadre, arresting or killing thousands of its veterans, its major leadership symbols survived intact, and those symbols, and their power to lead and to inspire, became Al Qaeda's most important asset.

After Tora Bora, the Qaeda fighters who survived regrouped in neighboring countries. "˜"˜We began to converge on Iran one after the other,'' Saif al-Adel recalled in a recent book by an Egyptian journalist. "˜"˜We began to form some groups of fighters to return to Afghanistan to carry out well-prepared missions there.'' It is these men, along with the reconstituted Taliban, that 16,000 American soldiers are still fighting today.

Not all the fighters would return to Afghanistan. Other targets of opportunity loomed on the horizon of the possible. "˜"˜Abu Mus'ab and his Jordanian and Palestinian comrades opted to go to Iraq,'' al-Adel recalled, for, he said, an "˜"˜examination of the situation indicated that the Americans would inevitably make a mistake and invade Iraq sooner or later. Such an invasion would aim at overthrowing the regime. Therefore, we should play an important role in the confrontation and resistance.''

Abu Mus'ab is Abu Mus'ab al-Zarqawi — or A.M.Z. to the American troops who are pursuing him and his Qaeda in Mesopotamia forces all over the shattered landscape of occupied Iraq. The United States, as Al Qaeda had hoped, had indeed come out of its hole.

V

It was strangely beautiful, the aftermath of the explosion in Baghdad: two enormous fires, bright orange columns of flame rising perhaps 20 feet into the air, and clearly discernible in the midst of each a cage of glowing metal: what remained of two four-wheel-drive vehicles. Before the flames, two bodies lay amid a scattering of glass and sand; the car bomb had toppled the sandbags piled high to protect the building, collapsing the facade and crushing a dozen people. It was Oct. 27, 2003, and I stood before what remained of the Baghdad office of the International Committee of the Red Cross. In the distance, I heard a second huge explosion, saw rising the great plume of oily smoke; within the next 45 minutes, insurgents attacked four more times, bombing police stations throughout the capital, killing at least 35. Simultaneity and spectacle: Qaeda trademarks. I was gazing at Zarqawi's handiwork.

Behind me, the press had gathered, a jostling crowd of aggressive, mostly young people bristling with lenses short and long, pushing against the line of young American soldiers, who, assault rifles leveled, were screaming at them to stay back. The scores of glittering lenses were a necessary part of the equation, transforming what in military terms would have been a minor engagement into a major defeat.

"˜"˜There is no war here,'' an American colonel told me a couple of days before in frustration and disgust. "˜"˜There's no division-on-division engagements, nothing really resembling a war. Not a real war anyway.''

It was not a war the Americans had been trained or equipped to fight. With fewer than 150,000 troops — and many fewer combat soldiers — they were trying to contain a full-blown insurgency in a country the size of California. The elusive enemy — an evolving, loose coalition of a score or so groups, some of them ex-Baathists from Saddam Hussein's dozen or so security agencies, some former Iraqi military personnel, some professional Islamic insurgents like Zarqawi, some foreign volunteers from Saudi Arabia or Kuwait or Syria come to take the jihad to the Americans — attacked not with tanks or artillery or infantry assaults but with roadside bombs and suicide car bombers and kidnappings. Iraq, bin Laden declared, had become a "˜"˜golden opportunity'' to start a "˜"˜third world war'' against "˜"˜the crusader-Zionist coalition.''

Amid the barbed wire and blast walls and bomb debris of post-occupation Iraq, you could discern a clear strategy behind the insurgent violence. The insurgents had identified the Americans' points of vulnerability: their international isolation; their forced distance, as a foreign occupier, from Iraqis; and their increasing disorientation as they struggled to keep their footing on the fragile, shifting, roiling political ground of post-Hussein Iraq. And the insurgents hit at each of these vulnerabilities, as Begin had urged his followers to do, "˜"˜deliberately, tirelessly, unceasingly.''

When, during the summer of 2003, the Bush administration seemed to be reaching out to the United Nations for political help in Iraq, insurgents struck at U.N. headquarters in Baghdad, killing the talented envoy Sergio Vieira de Mello and 21 others and driving the United Nations from the country. When the Americans seemed to be trying to attract Arab forces to come to Iraq to help, the insurgents struck at the Jordanian Embassy, killing 17. When the Turks offered to send troops, the insurgents bombed the Turkish Embassy. When nongovernmental organizations seemed the only outsiders still working to ease the situation in Iraq, insurgents struck at the Red Cross, driving it and most other nongovernmental organizations from the country.

Insurgents in Iraq and jihadists abroad struck America's remaining allies. First they hit the Italians, car-bombing their base in Nasiriyah in November 2003, killing 28. Then they struck the Spanish, bombing commuter trains in Madrid on March 11, 2004, killing 191. Finally they struck the British, bombing three London Underground trains and a double-decker bus this July, killing 56. It is as if the insurgents, with cold and patient precision, were severing one by one the fragile lines that connected the American effort in Iraq to the rest of the world.

With car bombs and assassinations and commando attacks, insurgents have methodically set out to kill any Iraqi who might think of cooperating with the Americans, widening the crevasse between occupiers and occupied. They have struck at water lines and electricity substations and oil pipelines, interrupting the services that Iraqis depended on, particularly during the unbearably hot summers, keeping electrical service in Baghdad far below what it was under Saddam Hussein — often only a few hours a day this summer — and oil exports 300,000 barrels a day below their prewar peak (helping to double world oil prices). Building on the chaotic unbridled looting of the first weeks of American rule, the insurgents have worked to destroy any notion of security and to make clear that the landscape of apocalyptic destruction that is Baghdad, with its omnipresent concrete blast walls and rolls of concertina wire and explosions and gunshots, should be laid at the feet of the American occupier, that unseen foreign power that purports to rule the country from behind concrete blast walls in the so-called Green Zone but dares to venture out only in tanks and armored cars.

"˜"˜With . . . officials attempting to administrate from behind masses of barbed wire, in heavily defended buildings, and . . . living in pathetic seclusion in "˜security zones,' one cannot escape the conclusion that the government . . . is a hunted organization with little hope of ever being able to cope with conditions in this country as they exist today.'' However vividly these words fit contemporary Baghdad, they are in fact drawn from the report of the American consul general in Jerusalem in 1947, describing what Begin's guerrilla forces achieved in their war against the British. "˜"˜The very existence of an underground,'' as Begin remarked in his memoirs, "˜"˜must, in the end, undermine the prestige of a colonial regime that lives by the legend of its omnipotence. Every attack which it fails to prevent is a blow to its standing.''

In Iraq, the insurgents have presided over a catastrophic collapse in confidence in the Americans and a concomitant fall in their power. It is difficult to think of a place in which terror has been deployed on such a scale: there have been suicide truck bombs, suicide tanker bombs, suicide police cars, suicide bombers on foot, suicide bombers posing as police officers, suicide bombers posing as soldiers, even suicide bombers on bicycles. While the American death toll climbs steadily toward 2,000, the number of Iraqi dead probably stands at 10 times that and perhaps many more; no one knows. Conservative unofficial counts put the number of Iraqi dead in the war at somewhere between 25,000 and 30,000, in a country a tenth the size of the United States.

Civil wars, of course, are especially bloody, and a civil war is now being fought in Iraq. The country is slowly splitting apart along the lines where French and British negotiators stitched it together early in the last century out of three Ottoman provinces — Mosul, Baghdad and Basra — and it is doing so with the enthusiastic help of the Islamists, who are doing all they can to provoke a Shia-Sunni regionwide war.

The Kurds in the north, possessed of their own army and legislature, want to secure what they believe are their historic rights to the disputed city of Kirkuk, including its oil fields, and be quit of Iraq. The Shia in the south, now largely ruled by Islamic party militias trained by the Iranians and coming under the increasingly strict sway of the clerics on social matters, are evolving their oil-rich mini-state into a paler version of the Islamic republic next door. And in the center, the Baathist elite of Saddam Hussein's security services and army — tens of thousands of well-armed professional intelligence operatives and soldiers — have formed an alliance of convenience with Sunni Islamists, domestic and foreign, in order to assert their rights in a unitary Iraq. They are in effective control of many cities and towns, and they have the burdensome and humiliating presence of the foreign occupier to thank for the continuing success of their recruitment efforts. In a letter to bin Laden that was intercepted by American forces in January 2004, Zarqawi asked: "˜"˜When the Americans disappear . . . what will become of our situation?''

As Zarqawi described in his letter and in subsequent broadcasts, his strategy in Iraq is to strike at the Shia — and thereby provoke a civil war. "˜"˜A nation of heretics,'' the Shia "˜"˜are the key element of change,'' he wrote. "˜"˜If we manage to draw them onto the terrain of partisan war, it will be possible to tear the Sunnis away from their heedlessness, for they will feel the weight of the imminence of danger.'' Again a strategy of provocation — which plays on an underlying reality: that Iraq sits on the critical sectarian fault line of the Middle East and that a conflict there gains powerful momentum from the involvement of neighboring states, with Iran strongly supporting the Shia and with Saudi Arabia, Kuwait, Jordan and Syria strongly sympathetic to the Sunnis. More and more, you can discern this outline in the chaos of the current war, with the Iranian-trained militias of the Shia Islamist parties that now control the Iraqi government battling Sunni Islamists, both Iraqi and foreign-born, and former Baathists.

In the midst of it all, increasingly irrelevant, are the Americans, who have the fanciest weapons but have never had sufficient troops, or political will, to assert effective control over the country. If political authority comes from achieving a monopoly on legitimate violence, then the Americans, from those early days when they sat in their tanks and watched over the wholesale looting of public institutions, never did achieve political authority in Iraq. They fussed over liberalizing the economy and writing constitutions and achieving democracy in the Middle East when in fact there was really only one question in Iraq, emerging again and again in each successive political struggle, most recently in the disastrously managed writing of the constitution: how to shape a new political dispensation in which the age-old majority Shia can take control from the minority Sunni and do it in a way that minimized violence and insecurity — do it in a way, that is, that the Sunnis would be willing to accept, however reluctantly, without resorting to armed resistance. This might have been accomplished with hundreds of thousands of troops, iron control and a clear sense of purpose. The Americans had none of these. Instead they relied first on a policy of faith and then on one of improvisation, driven in part by the advice of Iraqi exile "˜"˜friends'' who used the Americans for their own purposes. Some of the most strikingly ideological decisions, like abruptly firing and humiliating the entire Iraqi Army and purging from their jobs many hundreds of thousands of Baath Party members, seemed designed to alienate and antagonize a Sunni population already terrified of its security in the new Iraq. "˜"˜You Americans,'' one Sunni businessman said to me in Baghdad last February, shaking his head in wonder, "˜"˜you have created your own enemies here.''

The United States never used what authority it had to do more than pretend to control the gathering chaos, never managed to look clearly at the country and confront Iraq's underlying political dysfunction, of which the tyranny of Saddam Hussein was the product, not the cause. "˜"˜The illusionists,'' Ambassador John Negroponte's people called their predecessors, the officials of the Coalition Provisional Authority under L. Paul Bremer III. Now, day by day, the illusion is slipping away, and with it what authority the Americans had in Iraq. What is coming to take its place looks increasingly like a failed state.

VI

It is an oft-heard witticism in Washington that the Iraq war is over and that the Iranians won. And yet the irony seems misplaced. A truly democratic Iraq was always likely to be an Iraq led not only by Shia, who are the majority of Iraqis, but by those Shia parties that are the largest and best organized — the Supreme Council for the Islamic Revolution in Iraq and the Dawa Islamic Party — which happen to be those blessed by the religious authorities and nurtured in Iran. Nor would it be a surprise if a democratic Saudi Arabia turned out to be a fundamentalist Saudi Arabia and one much less friendly to the United States. Osama bin Laden knows this, and so do American officials. This is why the United States is "˜"˜friendly'' with "˜"˜apostate regimes.'' Democratic outcomes do not always ensure friendly governments. Often the contrary is true. On this simple fact depends much of the history of American policy not only in the Middle East but also in Latin America and other parts of the world throughout the cold war. Bush administration officials, for all their ideological fervor, did the country no favor by ignoring it.

In launching his new cold war, George W. Bush chose a peculiarly ideological version of cold-war history. He opted not for containment, the cautious, status quo grand strategy usually attributed to the late George F. Kennan, but for rollback. Containment, by which the United States determinedly resisted Soviet attempts to expand its influence, would have meant a patient, methodical search for terrorists, discriminating between those groups that threaten the United States and those that do not, pursuing the former with determined, practical policies that would have drawn much from the military and law-enforcement cooperation of our allies and that would have included an effective program of nonproliferation to keep weapons of mass destruction out of terrorist hands. Rollback, on the other hand, meant something quite different; those advocating it during the 1950's considered containment immoral, for it recognized the status quo: Communist hegemony in Eastern Europe and parts of Asia. They wanted instead to destroy Communism entirely by "˜"˜rolling back'' Communists from territory they had gained, as Gen. Douglas MacArthur did briefly and, it turned out, catastrophically, in North Korea, and as President Eisenhower refused to do when he declined to support the Hungarian revolutionaries against the Soviet invasion in 1956.

The original advocates of rollback lost that struggle. In this new cold war, the rollback advocates triumphed and adopted as the heart of their policy a high-stakes, metaphysical gamble to "˜"˜democratize the Middle East'' and thus put an end, once and for all, to terrorism. They relied on a "˜"˜domino theory'' in which the successful implantation of democracy in Iraq would lead to a "˜"˜democratic revolution'' across the region. The ambition of this idea is breathtaking; it depends on a conception of American power as virtually limitless and on an entirely fanciful vision of Iraqi politics, a kind of dogged political wish-fulfillment that no sober analysis could penetrate. Replacing any real willingness to consider whether a clear course existed between here and there, between an invasion and occupation of Iraq and a democratic Middle East, was, at bottom, the simple conviction that since the United States enjoyed a "˜"˜preponderance of power'' unseen in the world since the Roman Empire, and since its cause of democratic revolution was so incontrovertibly just, defeat was inconceivable. One detects here an echo of Vietnam: the inability to imagine that the all-powerful United States might lose.

American power, however, is not limitless. Armies can destroy and occupy, but it takes much more to build a lasting order, especially on the shifting sands of a violent political struggle: another Vietnam echo. Learning the lesson this time around may prove more costly, for dominoes can fall both ways. "˜"˜Political engineering on this scale could easily go awry,'' Stephen D. Biddle, a U.S. Army War College analyst, wrote this past April in a shrewd analysis. "˜"˜If a democratic Iraq can catalyze reform elsewhere, so a failed Iraq could presumably export chaos to its neighbors. A regionwide Lebanon might well prove beyond our capacity to police, regardless of effort expended. And if so, then we will have replaced a region of police states with a region of warlords and chronic instability. This could easily prove to be an easier operating environment for terrorism than the police states it replaces.''

The sun is setting on American dreams in Iraq; what remains now to be worked out are the modalities of withdrawal, which depend on the powers of forbearance in the American body politic. But the dynamic has already been set in place. The United States is running out of troops. By the spring of 2006, nearly every active-duty combat unit is likely to have been deployed twice. The National Guard and Reserves, meanwhile, make up an unprecedented 40 percent of the force, and the Guard is in the "˜"˜stage of meltdown,'' as Gen. Barry McCaffrey, retired, recently told Congress. Within 24 months, "˜"˜the wheels are coming off.'' For all the apocalyptic importance President Bush and his administration ascribed to the Iraq war, they made virtually no move to expand the military, no decision to restore the draft. In the end, the president judged his tax cuts more important than his vision of a "˜"˜democratic Middle East.'' The administration's relentless political style, integral to both its strength and its weakness, left it wholly unable to change course and to add more troops when they might have made a difference. That moment is long past; the widespread unpopularity of the occupation in Iraq and in the Islamic world is now critical to insurgent recruitment and makes it possible for a growing insurgent force numbering in the tens of thousands to conceal itself within the broader population.

Sold a war made urgent by the imminent threat of weapons of mass destruction in the hands of a dangerous dictator, Americans now see their sons and daughters fighting and dying in a war whose rationale has been lost even as its ending has receded into the indefinite future. A war promised to bring forth the Iraqi people bearing flowers and sweets in exchange for the beneficent gift of democracy has brought instead a kind of relentless terror that seems inexplicable and unending. A war that had a clear purpose and a certain end has now lost its reason and its finish. Americans find themselves fighting and dying in a kind of existential desert of the present. For Americans, the war has lost its narrative.

Of the many reasons that American leaders chose to invade and occupy Iraq — to democratize the Middle East; to remove an unpredictable dictator from a region vital to America's oil supply; to remove a threat from Israel, America's ally; to restore the prestige sullied on 9/11 with a tank-led procession of triumph down the avenues of a conquered capital; to seize the chance to overthrow a regime capable of building an arsenal of chemical and biological weapons — of all of these, it is remarkable that the Bush administration chose to persuade Americans and the world by offering the one reason that could be proved to be false. The failure to find the weapons of mass destruction, and the collapse of the rationale for the war, left terribly exposed precisely what bin Laden had targeted as the critical American vulnerability: the will to fight.

How that collapse, reflected in poll numbers, will be translated into policy is a more complicated question. One of 9/11's more obvious consequences was to restore to the Republicans the advantage in national security they surrendered with the cold war's end; their ruthless exploitation of this advantage and the Democrats' compromising embrace of the Iraq war has in effect left the country, on this issue, without an opposition party. Republicans, who fear to face the voters shackled to a leader whose approval ratings have slid into the low 40's, are the ones demanding answers on the war. The falling poll numbers, the approaching midterm elections and the desperate manpower straits of the military have set in motion a dynamic that could see gradual American withdrawals beginning in 2006, as Gen. George W. Casey Jr., the commander in Iraq, acknowledged publicly in July. Unless Iraq's political process, which has turned another downward spiral with Sunni negotiators' rejection of the constitution, can somehow be retrieved, American power in Iraq will go on deteriorating.

Two and a half years into the invasion, for U.S. policy in Iraq, the time of "˜"˜the illusionists'' has finally passed. Since the January elections, which Sunnis largely boycotted, American officials have worked hard to persuade Sunni leaders to take part in the constitutional referendum and elections, hoping thereby to isolate the Baathist and Islamist extremists and drain strength from the insurgency. This effort comes very late, however, when Iraqi politics, and the forces pulling the country apart, have taken on a momentum that waning American power no longer seems able to stop. Even as the constitutional drama came to a climax last month, the president telephoned Abdul Aziz Hakim, the Shia cleric who leads the Sciri Party, appealing for concessions that might have tempted the Sunnis to agree to the draft; the Shia politician, faced with the American president's personal plea, did not hesitate to turn him down flat. Perhaps the best hope now for a gradual American withdrawal that would not worsen the war is to negotiate a regional solution, which might seek an end to Sunni infiltration from U.S. allies in exchange for Shia guarantees of the Sunni position in Iraq and a phased American departure.

For all the newfound realism in the second-term administration's foreign policy, in which we have seen a willingness finally to negotiate seriously with North Korea and Iran, the president seems nowhere close to considering such an idea in Iraq, insisting that there the choice is simple: the United States can either "˜"˜stay the course'' or "˜"˜cut and run.'' "˜"˜An immediate withdrawal of our troops in Iraq, or the broader Middle East, as some have called for,'' the president declared last month, "˜"˜would only embolden the terrorists and create a staging ground to launch more attacks against America and free nations.'' These words, familiar and tired, offering no solution beyond staying a course that seems to be leading nowhere, have ceased to move Americans weary of the rhetoric of terror. That does not mean, however, that they may not be entirely true.

VII

We cannot know what future Osama bin Laden imagined when he sent off his 19 suicide terrorists on their mission four years ago. He got much wrong; the U.S. military, light years ahead of the Red Army, would send no tank divisions to Afghanistan, and there has been no uprising in the Islamic world. One suspects, though, that if bin Laden had been told on that day that in a mere 48 months he would behold a world in which the United States, "˜"˜the idol of the age,'' was bogged down in an endless guerrilla war fighting in a major Muslim country; a world in which its all-powerful army, with few allies and little sympathy, found itself overstretched and exhausted; in which its dispirited people were starting to demand from their increasingly unpopular leader a withdrawal without victory — one suspects that such a prophecy would have pleased him. He had struck at the American will, and his strategy, which relied in effect on the persistent reluctance of American leaders to speak frankly to their people about the costs and burdens of war and to expend the political capital that such frank talk would require, had proved largely correct.

He has suffered damage as well. Many of his closest collaborators have been killed or captured, his training camps destroyed, his sanctuary occupied. "˜"˜What Al Qaeda has lost,'' a senior Defense Department official said five months after the attacks, "˜"˜again, it's lost its center of gravity. . . . The benefits of Afghanistan cannot be overestimated. Again, it was the one state sponsor they had.'' This analysis seems now a vision of the past. Al Qaeda was always a flexible, ghostly organization, a complex worldwide network made up of shifting alliances and marriages of convenience with other shadowy groups. Now Al Qaeda's "˜"˜center of gravity,'' such as it is, has gone elsewhere.

In December 2003, a remarkable document, "˜"˜Jihadi Iraq: Hopes and Dangers,'' appeared on the Internet, setting out a fascinating vision of how to isolate the United States and pick off its allies one by one. The truly ripe fruit, concludes the author, is Spain: "˜"˜In order to force the Spanish government to withdraw from Iraq the resistance should deal painful blows to its forces . . . [and] make utmost use of the upcoming general election. . . . We think that the Spanish government could not tolerate more than two, maximum three blows, after which it will have to withdraw. . . . ''

Three months later, on March 11, 2004 — 3/11, as it has come to be known — a cell of North African terrorists struck at the Atocha Train Station in Madrid. One hundred ninety-one people died — a horrific toll but nowhere near what it could have been had all of the bombs actually detonated, simultaneously, and in the station itself. Had the terrorists succeeded in bringing the roof of the station down, the casualties could have surpassed those of 9/11.

In the event, they were quite sufficient to lead to the defeat of the Spanish government and the decision of its successor to withdraw its troops from Iraq. What seems most notable about the Madrid attack, however — and the attack on Jewish and foreign sites in Casablanca on May 17, 2003, among others — is that the perpetrators were "˜"˜home-grown'' and not, strictly speaking, Al Qaeda. "˜"˜After 2001, when the U.S. destroyed the camps and housing and turned off the funding, bin Laden was left with little control,'' Marc Sageman, a psychiatrist and former C.I.A. case officer who has studied the structure of the network, has written. "˜"˜The movement has now degenerated into something like the Internet. Spontaneous groups of friends, as in Madrid and Casablanca, who have few links to any central leadership, are generating sometimes very dangerous terrorist operations, notwithstanding their frequent errors and poor training.''

Under this view, Al Qaeda, in the form we knew it, has been subsumed into the broader, more diffuse political world of radical Salafi politics. "˜"˜The network is now self-organized from the bottom up and is very decentralized,'' Sageman wrote. "˜"˜With local initiative and flexibility, it's very robust.''

We have entered the era of the amateurs. Those who attacked the London Underground — whether or not they had any contact with Al Qaeda — manufactured their crude bombs from common chemicals (including hydrogen peroxide, bleach and drain cleaner), making them in plastic food containers, toting them to Luton Station in coolers and detonating them with cellphone alarms. One click on the Internet and you can pull up a Web site offering a recipe — or, for that matter, one showing you how to make a suicide vest from commonly found items, including a video download demonstrating how to use the device: "˜"˜There is a possibility that the two seats on his right and his left might not be hit with the shrapnel,'' the unseen narrator tells the viewer. Not to worry, however: "˜"˜The explosion will surely kill the passengers in those seats.''

During the four years since the attacks of 9/11, while terrorism worldwide has flourished, we have seen no second attack on the United States. This may be owed to the damage done Al Qaeda. Or perhaps planning and preparation for such an attack is going on now. When it comes to the United States itself, the terrorists have their own "˜"˜second-novel problem'' — how do you top the first production? More likely, though, the next attack, when it comes, will originate not in the minds of veteran Qaeda planners but from this new wave of amateurs: viral Al Qaeda, political sympathizers who nourish themselves on Salafi rhetoric and bin Laden speeches and draw what training they require from their computer screens. Very little investment and preparation can bring huge rewards. The possibilities are endless, and terrifyingly simple: rucksacks containing crude homemade bombs placed in McDonald's — one, say, in Times Square and one on Wilshire Boulevard, 3,000 miles away, exploded simultaneously by cellphone. The effort is small, the potential impact overwhelming.

Attacks staged by amateurs with little or no connection to terrorist networks, and thus no visible trail to follow, are nearly impossible to prevent, even for the United States, with all of its power. Indeed, perhaps what is most astonishing about these hard four years is that we have managed to show the world the limits of our power. In launching a war on Iraq that we have been unable to win, we have done the one thing a leader is supposed never to do: issue a command that is not followed. A withdrawal from Iraq, rapid or slow, with the Islamists still holding the field, will signal, as bin Laden anticipated, a failure of American will. Those who will view such a withdrawal as the critical first step in a broader retreat from the Middle East will surely be encouraged to go on the attack. That is, after all, what you do when your enemy retreats. In this new world, where what is necessary to go on the attack is not armies or training or even technology but desire and political will, we have ensured, by the way we have fought this forever war, that it is precisely these qualities our enemies have in large and growing supply.